Rule 1. Live within a budget
Rule 2. Try to pick a young offensive player that wasn't drafted in the first round. (This will allow for enough production of cards to make it challenging.)
Rule 3. Shoot for someone no one else is trying to hoard. (You don't want to be competing on cards and end up overpaying).

Hey, you might get lucky! As your new player goes into the final year of his rookie contract, he might explode and you can flip your dupes to cover your costs!